Semiconductor dies that contain radio circuits such as baseband and RF (Radio Frequency) ICs (Integrated Circuits) are difficult to package and integrate into very small electronic devices because the radio circuits generate heat and electromagnetic waves. Radio circuit dies are packaged in QFN (Quad Flat Non Lead) packages and other packages that have a solid metal plate to cover the back side of the die (backside metallization). The backside metallization serves as shield (e.g. an EMI (Electro-Magnetic Interference) shield) to protect other fast switching devices. The backside metallization also may serve as a heat sink for radio circuit dies that require high power dissipation such as PMICs (Power Management Integrated Circuits) and PAs (Power Amplifiers).
With the push for ever more miniaturization, dies and their packages are made ever thinner Currently dies may be thinned to a height of 50 μm or less. At those thicknesses, the circuitry of the semiconductor die is close enough to the backside metallization to be affected by it. A solid metal plate on the backside of the die becomes detrimental to any RF circuits such as PAs, LNAs (Low Noise Amplifier), VCOs (Voltage Controlled Oscillator), mixers, etc. . . . . The large metal plate decreases the inductance and the quality factor (Q) of any inductors or transformers on the die. These components are commonly used in radio circuits. The reduced inductance and Q may lead to frequency shifts, increased noise, and reduced output power. Without the backside metallization, the die may warp, crack, overheat, or suffer other problems.